The performance of the Planck instruments in space is enabled by their low operating temperatures, 20K for LFI and 0.1K for HFI, achieved
through a combination of passive radiative cooling and three active mechanical coolers. Active coolers were chosen to minimize straylight on the
detectors and to maximize lifetime. The scientific requirement for very broad frequency led to two detector technologies with widely dierent
temperature and cooling needs. This made use of a helium cryostat, as used by previous cryogenic space missions (IRAS, COBE, ISO, SPITZER,
AKARI), infeasible. Radiative cooling is provided by three V-groove radiators and a large telescope bae. The active coolers are a hydrogen
sorption cooler (<20 K), a 4He Joule-Thomson cooler (4.7 K), and a 3He-4He dilution cooler (1.4K and 0.1 K). The flight system was at ambient
temperature at launch and cooled in space to operating conditions. The bolometer plate of the High Frequency Instrument reached 93mK on 3
July 2009, 50 days after launch. The solar panel always faces the Sun, shadowing the rest of Planck, and operates at a mean temperature of 384 K.
At the other end of the spacecraft, the telescope bae operates at 42.3K and the telescope primary mirror operates at 35.9 K. The temperatures
of key parts of the instruments are stabilized by both active and passive methods. Temperature fluctuations are driven by changes in the distance
from the Sun, sorption cooler cycling and fluctuations in gas-liquid flow, and fluctuations in cosmic ray flux on the dilution and bolometer plates.
These fluctuations do not compromise the science data.